Rigid Outer-Layers to Reduce Cost
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Designing flex and rigid-flex can be tricky. Not only do you need to design for a product that will bend, fold and flex, you are working with different materials and fabrication processes. Plusm it is difficult to clearly communicate how the part should flex in final application. Typically, the goal is to use the most cost-effective stack up that meets both your electrical and mechanical performance needs. Logically, a 3-layer design should be less expensive than 4 layers. However, rigid flex is a funny thing. The following example proves that logic to be incorrect.
A specific medical application originally had a rigid, 3- layer, flex design. It had flexible layers on the outside with a rigid layer sandwiched between them. The original intention was to keep the layer count as low as possible to keep costs down..The stack up had a dense component area, supported by the rigid inner-layer. Its flex area needed dynamically flexing with a tight bend radius. Once the design was completed, it was sent to the fabricator for quotation. Pricing came back higher than expected, but this was one of the first rigid flex designs this company had done, so it was assumed this was the typical price of rigid flex.
As this project was going through tooling and set up at the fabricator, the designer was asked if flexible photoimageable solder mask could be used to help define the surface mount pads in the dense component area. Drilled polyimide coverlay was not able to support the density. This was approved, product was built, shipped and assembled. As the flex was mounted in the unit and powered up, it was quickly failed. Failure analysis showed the traces in the flex tail were cracking in use, causing the failure.
Working with the fabricator, it was obvious that the selected flexible solder mask was not flexible enough to withstand the end-use flexing requirements. This is a common issue. Design and fabrication rarely communicate with each other how the end application will use the flex. A a part of the problem is that Gerber files are two dimensional while the end use is three dimensional. Had the fabricator understood the need for dynamically flexing, they would not have recommended the flexible solder mask.
After several discussions, they decided to change the flexible solder mask to a polyimide coverlay, bringing us back to the initial issue with the surface mount pad density and definition. As a solution, the fabricator recommended laser drilling the polyimide coverlay. The two parties also discussed changing the fabrication to a button plate rather than a panel plate. Button plating only adds copper to the pad area and plated through-holes. This eliminates electrodeposited copper from the construction, keeping the layers as flexible as possible.
The project was again released for fabrication, assembled, installed and tested successfully. Perfect! Right?
Here is what was going on behind the scenes to fabricate this part number. Flex on the outer layers of a rigid flex construction is not common and in this case was significantly driving up costs. The fabricator to reduce the panel size, and subsequently the number of pieces running per panel, which also drove up the costs. In addition, the button plating process itself adds extra cost.
Ultimately, this product was redesigned as a more conventional, 4-layer rigid-flex construction with rigid layers on the outer layers and the flex layers sandwiched between. Those dense component areas could then use a standard solder mask process, eliminating the button plating process. Switching to a standard process also increased the panel size and the number per panel. Even though the raw material costs were higher, the overall cost of the part decreased by 20% with this 4-layer design.
First, the initial 3-layer rigid-flex design was ultimately successful, and functioned as needed. There was nothing “wrong” with that design. But, there was a less expensive approach; the rigid flex design. Lesson learned.
Could this have been avoided? Maybe. Part of the issue was the learning curve that goes along with new technology, and part of it was communication. I sometimes feel that I have become like a broken record, stuck on the same few words, but my best advice for designers new to rigid- flex is always to work with your fabricator, over-explain what you are trying to accomplish and ask for their advice. They work with the technology every day and are happy to recommend best practices and help avoid introducing unnecessary costs.